Synthesis, chemical ordering, and magnetic properties of self-assembled FePt–Ag nanoparticles

$[\mathrm{FePt}{]}_{\text{1−x}}{\mathrm{Ag}}_x$ nanoparticles were chemically synthesized. Self-assembled arrays were characterized by transmission electron microscopy and small angle x-ray diffraction (XRD). Particles were annealed at temperatures from 300 to 500 °C for 30 min and degree of chemical ordering was characterized by large angle XRD and by magnetometry. Compared with pure FePt nanoparticles, additive Ag reduced the A1 to ${\text{L1}}_{\mathrm{0}}$ ordering temperature by more than 100 °C. A maximum coercivity of more than 10 kOe was measured for samples with $\text{∼15\%}$ Ag annealed at 500 °C, compared with about 2 kOe for samples without Ag. This reduction in required annealing temperature significantly reduces particle coalescence and loss in positional order. XRD measurements suggest that the reduction of the ordering temperature is due to defects and lattice strain introduced by the Ag and the subsequent segregation of the Ag upon annealing, activating the nucleation of the ordered phase.